1. Field of the Invention
The present invention relates to a radiographic image correcting apparatus, for removing noise from radiographic image data sets, which are obtained from stimulable phosphor sheets.
2. Description of the Related Art
When certain types of phosphors are irradiated with radiation (X-rays, α-rays, β-rays, γ-rays, electron beams, ultraviolet rays, and the like), a portion of the radiation energy accumulates within the phosphors. When excitation light, such as visible light, is irradiated onto the phosphors, the phosphors emit stimulated phosphorescence corresponding to the energy accumulated therein. Phosphors having this property are referred to as stimulable phosphors. Image readout apparatuses that utilize these stimulable phosphors have been proposed. Radiographic image information of a subject, such as a human body, is temporarily recorded in a stimulable phosphor sheet. Then, excitation light is irradiated onto the stimulable phosphor sheet to cause stimulated phosphorescence to be emitted. The image readout apparatuses photoelectrically read out the stimulated phosphorescence to obtain image signals.
There is a problem that noise generated due to the stimulable phosphor sheet is unavoidable in the radiographic image information obtained by the image readout apparatuses. The noise generated due to the stimulable phosphor sheet includes: X-ray quantum noise, light quantum noise, and fixed noise (Refer to Eiji Ogawa et al., “Quantitative Analysis of Imaging Performance for Computed Radiography Systems”, SPIE, Vol. 2432, pp. 421-431, 1995).
Here, the X-ray quantum noise and the light quantum noise are random noise that depends on the X-ray dosage and the amount of light emission accompanying X-ray irradiation. The X-ray quantum noise and the light quantum noise have an inverse proportionate relationship with the X-ray dosage. That is, X-ray quantum noise and light quantum noise are prevalent at low X-ray dosage regions, and minimal at high X-ray dosage regions. On the other hand, the fixed noise is mainly unique noise inherent to the sheet. The fixed noise is caused by differences in the amounts of emitted light at different positions on the surface of the sheet. The fixed noise influences image quality at the practical dosage regions and at high dosage regions. Fixed noise having comparatively long periods (low frequencies) are referred to as “sheet irregularities”. However, the frequency of the fixed noise is not limited to low frequencies. Fixed noise having high frequencies (for example, 0.5 c/mm to the Nyquist frequency) also exists. The fixed noise having the range of low to high frequencies is collectively referred to as “structural noise” (or structural mottle). Each sheet possesses a different noise pattern, and the noise appears as graininess in images.
Japanese Unexamined Patent Publication No. 2000-013599 discloses a method for removing the structural noise. In this method, first, a solid image is obtained from a stimulable phosphor sheet, which has undergone solid exposure (uniform exposure). Then, radiographic images are corrected based on the solid image.
Japanese Unexamined Patent Publication No. 2001-166404 discloses another method for removing the structural noise. In this method, the relationship between the amount of excitation light, which is transmitted through a stimulable phosphor sheet when the excitation light is irradiated thereon, and the amount of stimulated phosphorescence emitted by the stimulable phosphor sheet when the excitation light is irradiated thereon is obtained. Then, actual images are corrected, based on this relationship.
However, in the method disclosed in Japanese Unexamined Patent Publication No. 2000-013599, the solid image, which is the reference for correction, includes both structural noise and quantum noise. Therefore, there is a problem that the accuracy in correction of the structural noise is poor.
In the method disclosed in Japanese Unexamined Patent Publication No. 2001-166404, a photodetecting means for detecting the amount of excitation light which is transmitted through the stimulable phosphor sheet is necessary. Therefore, there is a problem that the apparatus becomes large.